This invention relates to a process and apparatus for producing low sulfur content hot reducing gas and especially that gas formed by the combustion and gasification of sulfur-bearing carbonaceous fuel. The gasification of solid carbonaceous fuel, such as by reaction with a limited quantity of oxygen to produce carbon monoxide, is well known. Either pure oxygen or air, with or without steam, may be utilized in the reaction. The products of combustion are reducing gases including carbon monoxide and usually hydrogen. Carbon dioxide, water vapor and nitrogen may also be present in the reducing gas. Hydrogen is produced from the hydrocarbons in the fuel, and also by reaction of injected steam with carbon, while nitrogen may be brought in by air or may also be contained in the fuel. Carbon dioxide and water vapor may also be used in addition to react with the carbonaceous fuel and the reducing gas produced therefrom a vary the final composition of the product reducing gas.
One of the serious problems with gasification of carbonaceous fuels is that many commercially available carbonaceous fuels contain sulfur. Sulfur-containing reducing gases, usually predominantly hydrogen sulfide, are produced when these fuels are reacted with air or oxygen in a gasification process. These sulfur-containing gases in the reducing gas are often objectionable for a number of reasons. One of the chief reasons is, of course, that when the sulfur finally ends up in the air, it results in serious air pollution problems. Additionally, this sulfur should be removed from the product gas before its use in many applications, such as metallurgical reducing gas, synthesis gas, or feed stock for pipeline gas. Also, if the product gas is burnt to raise steam or generate electricity, it is advantageous to remove the H.sub.2 S before combustion rather than having to remove SO.sub.2 from the larger volume of combusted gas. In at least two of these applications, i.e. as a reducing gas for direct reduction of iron ore or fuel for gas-turbine engines, it is desirable to remove the H.sub.2 S while the product gas is still hot so that the gas can be used directly without loss of heat values.
In order to balance, if only partly, the cost of desulfurizing the hot reducing gases, the byproducts of sulfur removal should be marketable: (i) recovery of sulfur from the spent absorbent, (ii) regeneration of absorbent for recycle, or (iii) marketing the treated spent absorbent, after the recovery of sulfur, for other applications. The absorbent used for desulfurization of hot reducing gases should have the capability of lowering the sulfur content of the treated gas to levels below 100 ppm without much changing the reducing capacity, hence fuel value, of the gas.
Attempts have been made to remove the sulfur during the gasification reaction itself. U.S. Pat. No. 3,533,730, incorporated herein by reference, is an example of such a process whereby the carbonaceous fuel is reacted with a controlled quantity of oxygen beneath the surface of a molten iron bath and whereby lime on the surface of the molten iron bath is used to desorb sulfur from the iron bath. Sulfur is then recovered from the coal ash-lime-sulfur molten slag byproduct. These are, however, serious questions concerning the practical operability of this process. The rate of coal gasification depends upon the rate of coal dissolution for a given melt size. Furthermore, the sulfur in the slag byproduct is recovered only by costly additional steps. The gasification product generally contains fly ash which also requires an extra step for removal.
The use of calcined dolomite has been suggested for a regenerative cycle process of desulfurization of hot reducing gases. See U.S. Pat. Nos. 3,276,203; 3,296,775; 3,307,350; 3,402,998; and 3,853,538, each incorporated herein by reference. While dolomite is an effective gas-desulfurizing agent, the most commonly proposed method of regenerating dolomite, reacting with CO.sub.2 and H.sub.2 O under slightly reducing conditions at pressures greater than about 50 psig and temperatures preferably about 1000.degree.-1200.degree. F. to liberate H.sub.2 S, does not achieve complete regeneration of the dolomite. One of the problems is that calcium carbonate formed in the regeneration coats the regenerated dolomite, thereby reducing its effectiveness. Furthermore, because the spent dolomite contains appreciable nonregenerated calcium sulfide, it must undergo expensive and complete treatment to bring it to a state suitable for disposal without causing pollution of the air and groundwater. When dolomite is calcined after having been regenerated by the above suggested process, some of the residual sulfur in the dolomite can be released, which requires difficult treatment to bring the stack gas to a condition suitable for venting to the atmosphere.
The use of porous manganese oxide pellets has been disclosed for use in another regenerative cycle process for desulfurizing hot reducing gas. See U.S. Pat. No. 4,164,544, incorporated herein by reference. The system of this patent is even more efficient than the dolomite system and has the ability to regenerate the spent manganese oxide pellet and reuse it many times without loss of reactivity.
However, in both the dolomite and manganese oxide systems described above the presence of fly ash in the reducing gas seriously interferes with the ability to regenerate the spent reactant. Since fly ash is generally present in reducing gases produced by gasification of coal or other carbonaceous material, this is a serious problem.
In the coal gasification process described in U.S. Pat. No. 3,625,164, incorporated herein by reference, crushed limestone is mixed with coal and gasification by partial combustion takes place in a fluidized bed. The calcium oxide of the limestone acts as a desulfurizing agent in the process. The resulting calcium sulfide together with coal ash produced as byproducts in the process are discharged from the gasification zone by a moving grate. One of the methods for recovering elemental sulfur from the calcium sulfide is to quench the hot solid byproducts from the gasification zone in a body of water maintained at atmospheric conditions. Under these conditions, H.sub.2 S and water vapor are released and calcium hydroxide and inerts are recovered as solids. However, this process of sulfur removal is generally very sluggish and incomplete such that the ash residue still contains considerable sulfur and presents a serious disposal problem.